Low-noise amplifiers for integrated multi-mode direct-conversion receivers

Abstract

The evolution of wireless telecommunication systems during the last decade has been rapid. During this time the design driver has shifted towards fast data applications instead of speech. In addition, the different systems may have a limited coverage, for example, limited to urban areas only. Thus, it has become important for a mobile terminal to be able to use different wireless systems, depending on the application chosen and the location of the terminal.

The choice of receiver architecture affects the performance, size, and cost of the receiver. The superheterodyne receiver has hitherto been the dominant radio architecture, because of its good sensitivity and selectivity. However, superheterodyne receivers require expensive filters, which, with the existing technologies, cannot be integrated on the same chip as the receiver. Therefore, architectures using a minimum number of external components, such as direct conversion, have become popular. In addition, compared to the superheterodyne architecture, the direct-conversion architecture has benefits when multi-mode receivers, which are described in this thesis, are being designed.

In this thesis, the limitations placed on the analog receiver by different system specifications are introduced. The estimations for the LNA specifications are derived from these specifications. In addition, the limitations imposed by different types of receiver architectures are described. The inductively-degenerated LNA is the basis for all the experimental circuits. The different components for this configuration are analyzed and compared to other commonly-used configurations in order to justify the use of an inductively-degenerated LNA. Furthermore, the design issues concerning the LNA-mixer interface in direct-conversion receivers are analyzed. Without knowing these limitations, it becomes difficult to understand the choices made in the experimental circuits.

One of the key parts of this thesis describes the design and implementation of a single-chip multi-mode LNA, which is one of the key blocks in multi-mode receivers. The multi-mode structures in this thesis were developed for a direct-conversion receiver where only one system is activated at a time. The LNA interfaces to a pre-select filter and mixers and the different LNA components are analyzed in detail. Furthermore, the design issues related to possible interference from additional systems on single-chip receivers are analyzed and demonstrated.

A typical receiver includes variable gain, which can be implemented both in the analog baseband and/or in the RF. If the variable gain is implemented in the RF parts, it is typically placed in the LNA or in a separate gain control stage. Several methods that can be used to implement a variable gain in the LNA are introduced and compared to each other. Furthermore, several of these methods are included in the experimental circuits.

The last part of this thesis concentrates on four experimental circuits, which are described in this thesis. The first two chips describe an RF front-end and a direct-conversion receiver for WCDMA applications. The whole receiver demonstrates that it is possible to implement A/D converters on the same chip as sensitive RF blocks without significantly degrading receiver performance. The other two chips describe an RF front-end for WCDMA and GSM900 applications and a direct-conversion receiver for GSM900, DCS1800, PCS1900 and WCDMA systems. These ICs demonstrate the usability of the circuit structure developed and presented in this thesis. The chip area in the last multi-mode receiver is not significantly increased compared to corresponding single-system receivers.